Methods and compositions for preparing cardiomyocytes from stem cells and uses thereof

ABSTRACT

The present invention discloses novel compositions and methods for enhancing cardiac differentiation efficiency of stem cells or promoting ventricular and atrial cardiomyocytes formation from stem cells. The present invention also discloses the atrial and ventricular cardiomyocytes formed from the stem cells, and the uses of the cardiomyocytes for repairing cardiac injuries and screening for new medicaments for treating cardiac injuries.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase of PCT applicationPCT/CN2010/078645 having an international filing date of Nov. 11, 2010,entitled “Methods and Compositions for Preparing Cardiomyocytes FromStem Cells and Uses Thereof,” which claims priority to ChineseApplication No. 201010207603.0 filed Jun. 13, 2010, entitled “Methodsfor Efficiently Differentiating Stem Cells into Atrial and VentricularCardiomyocytes.” The contents of these applications are incorporatedherein by this reference in their entireties.

II. TECHNICAL FIELD

The present invention relates to novel compositions and methods forenhancing cardiac differentiation efficiency of stem cells and forpromoting atrial and ventricular cardiomyocytes formation from stemcells, the atrial and ventricular cardiomyocytes formed from the stemcells, and the uses of the cardiomyocytes, e.g., for cardiac injuryrepair and screening for new therapeutics for treating cardiac injuries.

III. BACKGROUND OF THE INVENTION

Using current technologies, embryonic atrial-, ventricular- andnodal-like cardiomyocytes can be differentiated non-specifically fromhESCs⁶⁻⁹. Swine transplantation studies show that implanted hESC derivedcardiomyocytes have pace making activities, a potential cause ofventricular arrhythmias¹⁰. The application of hESCs in myocardial repairis hampered by this cardiac subtype heterogeneity of hESC-derivedcardiomyocytes¹. To direct differentiation of hESCs into a desiredcardiac subtype, the mechanisms of cardiac subtype specification have tobe uncovered. Although several growth factors, such as activin A, bonemorphogenetic protein 4 (BMP4), wnt-3a, basic fibroblast growth factor(bFGF) and dickkopf homolog 1 (DKK1), have been identified to promotecardiogenesis, and are used in several hESC cardiac differentiationprotocols^(7,8,11), there is no evidence up to date showing that theseor other growth factors regulate the cardiac subtype specificationduring hESC differentiation. Identifying the key regulators of cardiacsubtype specification is critical for reducing the heterogeneity ofhESC-derived cardiomyocyte population which will be important for itslater use in regenerative medicine or as drug test systems^(6,12).

Bone morphogenetic protein (BMP) signaling is tightly controlled duringmesoderm and heart development. In mouse embryos, the BMP antagonistNoggin is transiently but strongly expressed in the cardiac crescent atembryo day E7.5 to E8.0¹³. Dkk1, a potent inducer of heartdevelopment^(8,14 15) synergizes with BMP antagonism to specify hearttissue in non-cardiogenic mesoderm from Xenopus embryos¹⁶. It has beenshown that long term treatment of hESCs with BMP4 inducestrophoblast-like cell differentiation¹⁷, while short term treatmentinitiates mesoderm formation¹⁸. Together, these results suggest thatinhibiting BMP signaling after mesoderm formation facilitates cardiacdevelopment.

Retinoic acid (RA) signaling not only restricts the cardiac progenitorpool, and exposure of the anterior lateral plate mesoderm of zebrafishembryos to the RA antagonist BMS-189453 causes uncommitted lateralmesodermal cells to become myocardial progenitors¹⁹, but also regulatesanterior-posterior polarization of the heart²⁰. Chicken transplantationstudies have revealed that the cardiogenic mesoderm from HH stages 4-6,originally fated to be atria, is competent to develop into functionalventricles and vice versa^(21,22). RA treatment of HH stage 4cardiogenic tissue activates the expression of the atrium-specific geneAMHC1 in anterior progenitors fated to develop into out-flow tracktissues²³. Furthermore, in both mouse and chicken embryos, inhibition ofRA signaling within critical periods produces embryos with oversizedventricles and smaller or missing atria, and exogenous addition of RAresults in reverted phenotypes^(5,24). Furthermore, studies with mouseembryonic stem cells indicated retinoic acid promotes the expression ofatrial related genes²⁵.

IV. DISCLOSURE OF THE INVENTION

In one aspect, the present disclosure provides a method for enhancingcardiac differentiation efficiency of a stem cell, which methodcomprises inhibiting BMP signaling after the initiation ofdifferentiation, or cardiac differentiation, of the stem cell. In onespecific embodiment, the present disclosure provides a method forenhancing cardiac differentiation efficiency of a stem cell, whichmethod comprises contacting a stem cell that has differentiated to formmesoderm with a bone morphogenetic protein (BMP) antagonist, whereby thecardiac differentiation efficiency of the stein cell contacted with theBMP antagonist is higher than the cardiac differentiation efficiency ofthe stem cell not contacted with the BMP antagonist. The cardiomyocytesproduced by the above method are also provided. A composition comprisinga stem cell that has differentiated to form mesoderm and treated with anexogenous BMP antagonist is further provided.

In another aspect, the present disclosure provides a method forpromoting ventricular cardiomyocyte formation from a stem cell, whichmethod comprises inhibiting retinoic acid signaling pathway in a stemcell that has differentiated to form mesoderm. Ventricularcardiomyocytes produced by the above method are also provided. Acomposition comprising a stem cell that has differentiated to formmesoderm and treated with an exogenous agent that inhibits retinoic acidsignaling pathway in the stem cell is further provided.

In still another aspect, the present disclosure provides a method forpromoting atrial cardiomyocyte formation from a stem cell, which methodcomprises stimulating or not inhibiting retinoic acid signaling pathwayin a stem cell that has differentiated to form mesoderm. Atrialcardiomyocytes produced by the above method are also provided. Acomposition comprising a stem cell that has differentiated to formmesoderm and treated with an exogenous agent that stimulates retinoicacid signaling pathway in the stem cell is further provided.

In yet another aspect, the present disclosure provides a method forgenerating a ventricular cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with an agent, e.g., bFGF and BMP4, to initiate stem cell differentiation; 2) contacting the stem celltreated by the agent, e.g., bFGF and BMP 4, with another agent, e.g.,activin A, to form mesoderm; 3) contacting the stem cell that hasdifferentiated to form mesoderm with a BMP antagonist such as Noggin, toenhance cardiac differentiation efficiency of the stem cell; 4)inhibiting retinoic acid signaling pathway in the stem cell treated byBMP antagonist, e.g., Noggin, to promote ventricular cardiomyocyteformation; and 5) contacting the stem cell treated by BMP antagonist,e.g., Noggin, with a wnt inhibitor, such as DKK1, to differentiate thestem cell into a ventricular cardiomyocyte. Ventricular cardiomyocytesproduced by the above method are also provided.

In yet another aspect, the present disclosure provides a method forgenerating a ventricular cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with bFGF and BMP 4; 2) contactingthe stem cell treated by bFGF and BMP 4 with activin A; 3) contactingthe stem cell that has been treated by activin A with Noggin; 4)inhibiting retinoic acid signaling pathway in the stem cell treated byNoggin; and 5) contacting the stem cell treated by Noggin with DKK1.Ventricular cardiomyocytes produced by the above method are alsoprovided.

In yet another aspect, the present disclosure provides a method forgenerating an atrial cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with an agent, e.g., bFGF and BMP4, to initiate stem cell differentiation; 2) contacting the stem celltreated by the agent, e.g., bFGF and BMP 4, with another agent, e.g.,activin A, to form mesoderm; 3) contacting the stem cell that hasdifferentiated to form mesoderm with a BMP antagonist, such as Noggin,to enhance cardiac differentiation efficiency of the stem cell; 4)stimulating or not inhibiting retinoic acid signaling pathway in thestem cell treated by Noggin to promote atrial cardiomyocyte formation;and 5) contacting the stem cell treated by Noggin with DKK1 todifferentiate the stem cell into an atrial cardiomyocyte. Atrialcardiomyocytes produced by the above method are also provided.

In yet another aspect, the present disclosure provides a method forgenerating an atrial cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with bFGF and BMP 4; 2) contactingthe stem cell treated by bFGF and BMP 4 with activin A; 3) contactingthe stem cell that has been treated by activin A with Noggin; 4)stimulating or not inhibiting retinoic acid signaling pathway in thestem cell treated by Noggin; and 5) contacting the stem cell treated byNoggin with DKK1. Atrial cardiomyocytes produced by the above method arealso provided.

In yet another aspect, the present disclosure provides a pharmaceuticalcomposition for treating a cardiac injury or disorder, whichpharmaceutical composition comprises an effective amount of thecardiomyocytes produced by the above methods, and optionally apharmaceutically acceptable carrier or excipient.

In yet another aspect, the present disclosure provides a method fortreating a cardiac injury or disorder in a subject, e.g., a human, whichmethod comprises administering, to a subject to which such treatment isneeded or desirable, an effective amount of the above pharmaceuticalcomposition.

In yet another aspect, the present disclosure provides a method foridentifying a modulator of a cardiomyocyte, which method comprises: 1)contacting a cardiomyocyte produced by the above methods with a testsubstance and measuring the effect of the test substance on a propertyof the cardiomyocyte; 2) measuring the property of the cardiomyocyte notcontacted with the test substance; whereby the property of thecardiomyocyte contacted with the test substance is different from thatof the cardiomyocyte not contacted with the test substance identifiesthe test substance as a modulator, e.g., a stimulator or inhibitor, ofthe property of the cardiomyocyte.

V. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows that Noggin (Ngn) and RA inhibition promote hESC cardiacdifferentiation. (A) An outline of the protocol used for thedifferentiation of human ESCs to cardiac lineages. (B) Frequencies ofCTNT⁺ cells at day 14; cardiac-induced cultures with addition of Ngn atthe time intervals indicated; the basic protocol (BP) was used as acontrol. (C) Frequencies of CTNT⁺ cells in day 14 cardiac-inducedcultures with addition of the RA inhibitor, RAi at the time intervalsindicated; BP was used as a control. (D) Flow cytometry analysis ofCTNT⁺ cells in cultures with BP as a control; BP cultures with Ngn addedat days 4 and 5, and BP cultures with Ngn added at days 4 and 5, and RAiadded from day 6 to day 8 (Ngn+RAi). (E) Quantitative RT-PCR geneexpression analysis of day 14 BP, Ngn-treated, and Ngn+RAi-treatedcultures, as indicated in (D) The average expression, normalized toGADPH, is shown. (F) Immunostaining analysis of cardiac-inducedcultures. Where shown, bars represent the standard error of the mean ofthree independent experiments. CM, conditioned medium. P compared withthe BP control or otherwise indicated, *P<0.05, **P<0.005, ***P<0.0005.

FIG. 2 shows morphologies and beating rates of differentiatedcardiomyocytes. (A) Flow cytometry analysis of CTNT⁺ cells from day 14cultures, differentiated with the BP, plus treatments with Ngn at days 4and 5, and RA (Ngn+RA) or RAi (Ngn+RAi.) treatment from days 6 to 8. (B)Statistics of the size of single cardiomyocytes from 60 day-old culturestreated with Ngn+RA (n=35) and Ngn+RAi (n=31), measured with ImageJsoftware. (C) Statistics of the beating rates (beat/minute) of 60day-old differentiated cultures (n=4). (D) Immunostaining of singlecells from 60 day-old differentiated cultures.

FIG. 3 shows characterization of hESC-derived cardiomyocytes, induced byNgn+RA and Ngn+RAi. (A) Quantitative RT-PCR analysis of the kinetics ofirx4 gene expression in Ngn+RA and Ngn+RAi cultures. The averageexpression, normalized to GAPDH, is shown. Where shown, bars representthe standard error of the mean of three independent experiments.***P<0.0005. (B) Immunostaining of 60 day-old Ngn+RA and Ngn+RAi inducedcultures, demonstrated MLC-2V expression in the majority of CTNT⁺ cellsof Ngn+RAi treated cultures but not in those of Ngn+RA treated cultures.(C) Western blotting of 60 day-old Ngn+RA and Ngn+RAi treated cultures,indicated that even though CTNT is evenly expressed in both cultures,MLC-2V is strongly expressed in Ngn+RAi-treated cultures but not inNgn+RA treated cultures.

FIG. 4 shows AP morphologies and Ca2+ release properties ofcardiomyocytes induced by Ngn+RA and Ngn+RAi. (A) Three major types ofAPs were observed in hESC-derived cardiomyoctes: nodal-like,atrial-like, and ventricular-like. The duration of the ventricular-likeAPs was greatly reduced by application of nifedipine (B, left); Nosignificant effect of nifedipine on atrial-like AP duration was observed(B, right). (C) the percentages of AP types recorded from Ngn+RAi andNgn+RA cultures are indicated. (D) Typical Ca²⁺ images recorded fromcardiomyocytes in Ngn+RA cultures (left) and Ngn+RAi cultures (right).Fluorescence profiles (bottom) were taken from the images (at positionsindicated by arrows). The properties of typical Ca²⁺ release eventsrecorded from cardiomyocytes in Ngn+RA and Ngn+RAi-treated cultures aresummarized in e (98 sparks from 18 cells in Ngn+RA-treated cultures, and348 sparks from 14 cells in Ngn+RAi-treated cultures were measured).*P<0.05 compared with Ngn+RAi cultures. All cells used inelectrophysiological studies were 60 to 90 days old. N-like, nodal likeAP; A-like, atrial like AP; V-like, ventricular-like AP.

FIG. 5 shows MLC-2v expression in different retinoid treated cultures.(A) MLC-2v and cTNT double staining for 60 day old cultures treated withNoggin+RA, Noggin alone and Noggin+RAi. (B) Western blots for MLC-2v,cTNT and β-actin with 60 day old cultures treated with Noggin+RA, Nogginalone and Noggin+RAi.

FIG. 6 shows cardiac gene expression in different treated cultures.Western blots for MLC-2a, ANF, β-MHC and β-actin with 60 day oldcultures treated with Noggin+RA, Noggin alone and Noggin+RAi.

VI. DETAILED DESCRIPTION OF THE INVENTION A. Definitions

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as is commonly understood by one of ordinary skillin the art to which this invention belongs. All patents, patentapplications (published or unpublished), and other publications referredto herein are incorporated by reference in their entirety. If adefinition set forth in this section is contrary to or otherwiseinconsistent with a definition set forth in the patents, applications,published applications and other publications that are hereinincorporated by reference, the definition set forth in this sectionprevails over the definition that is incorporated herein by reference.

As used herein, “a” or “an” means “at least one” or “one or more.”

As used herein, “mammal” refers to any of the mammalian class ofspecies. Frequently, the term “mammal,” as used herein, refers tohumans, human subjects or human patients.

As used herein, “an effective amount of a compound for treating aparticular disease” is an amount that is sufficient to ameliorate, or insome manner reduce the symptoms associated with the disease. Such amountmay be administered as a single dosage or may be administered accordingto a regimen, whereby it is effective. The amount may cure the diseasebut, typically, is administered in order to ameliorate the symptoms ofthe disease. Repeated administration may be required to achieve thedesired amelioration of symptoms.

As used herein, “treatment” means any manner in which the symptoms of acondition, disorder or disease are ameliorated or otherwise beneficiallyaltered. Treatment also encompasses any pharmaceutical use of thecompositions herein.

As used herein, “amelioration” of the symptoms of a particular disorderby administration of a particular pharmaceutical composition refers toany lessening, whether permanent or temporary, lasting or transient thatcan be attributed to or associated with administration of thecomposition.

As used herein, “production by recombinant means” refers to productionmethods that use recombinant nucleic acid methods that rely on wellknown methods of molecular biology for expressing proteins encoded bycloned nucleic acids.

As used herein, the term “subject” is not limited to a specific speciesor sample type. For example, the term “subject” may refer to a patient,and frequently a human patient. However, this term is not limited tohumans and thus encompasses a variety of mammalian species.

As used herein, “pharmaceutically acceptable salts, esters or otherderivatives” include any salts, esters or derivatives that may bereadily prepared by those of skill in this art using known methods forsuch derivatization and that produce compounds that may be administeredto animals or humans without substantial toxic effects and that eitherare pharmaceutically active or are prodrugs.

As used herein, a “prodrug” is a compound that, upon in vivoadministration, is metabolized or otherwise converted to thebiologically, pharmaceutically or therapeutically active form of thecompound. To produce a prodrug, the pharmaceutically active compound ismodified such that the active compound will be regenerated by metabolicprocesses. The prodrug may be designed to alter the metabolic stabilityor the transport characteristics of a drug, to mask side effects ortoxicity, to improve the flavor of a drug or to alter othercharacteristics or properties of a drug. By virtue of knowledge ofpharmacodynamic processes and drug metabolism in vivo, those of skill inthis art, once a pharmaceutically active compound is known, can designprodrugs of the compound (see, e.g., Nogrady (1985) Medicinal ChemistryA Biochemical Approach, Oxford University Press, New York, pages388-392).

As used herein, “test substance (or candidate compound)” refers to achemically defined compound (e.g., organic molecules, inorganicmolecules, organic/inorganic molecules, proteins, peptides, nucleicacids, oligonucleotides, lipids, polysaccharides, saccharides, orhybrids among these molecules such as glycoproteins, etc.) or mixturesof compounds (e.g., a library of test compounds, natural extracts orculture supernatants, etc.) whose effect on PTH antagonist is determinedby the disclosed and/or claimed methods herein.

As used herein, high-throughput screening (HTS) refers to processes thattest a large number of samples, such as samples of diverse chemicalstructures against disease targets to identify “hits” (see, e.g.,Broach, et al., High throughput screening for drug discovery, Nature,384:14-16 (1996); Janzen, et al., High throughput screening as adiscovery tool in the pharmaceutical industry, Lab Robotics Automation:8261-265 (1996); Fernandes, P. B., Letter from the society president, J.Biomol. Screening, 2:1 (1997); Burbaum, et al., New technologies forhigh-throughput screening, Curr. Opin. Chem. Biol., 1:72-78 (1997)). HTSoperations are highly automated and computerized to handle samplepreparation, assay procedures and the subsequent processing of largevolumes of data.

B. Methods and Compositions for Enhancing Cardiac DifferentiationEfficiency of Stem Cells and Cardiomyocyte Produced Thereof

In one aspect, the present disclosure provides a method for enhancingcardiac differentiation efficiency of a stem cell, which methodcomprises contacting a stem cell that has differentiated to formmesoderm with a bone morphogenetic protein (BMP) antagonist, whereby thecardiac differentiation efficiency of the stem cell contacted with theBMP antagonist is higher than the cardiac differentiation efficiency ofthe stem cell not contacted with the BMP antagonist. The cardiomyocytesproduced by the above method are also provided. A composition comprisinga stem cell that has differentiated to form mesoderm and treated with anexogenous BMP antagonist is further provided.

The present method can be used to enhance cardiac differentiationefficiency of any suitable stem cell. For example, the present methodcan be used to enhancing cardiac differentiation efficiency of atotipotent, pluripotent, multipotent, oligopotent or unipotent stemcell. In another example, the present method can be used to enhancingcardiac differentiation efficiency of an embryonic stem cell, an inducedpluripotent stem cell, a fetal stem cell or an adult stem cell. In stillanother example, the present method can be used to enhancing cardiacdifferentiation efficiency of a mammalian stem cell such as a human stemcell. In still another example, the present method can be used toenhancing cardiac differentiation efficiency of a human embryonic stemcell or a human induced pluripotent stem cell.

The stem cells can be obtained, prepared and/or maintained by anysuitable methods. For example, mouse ES cells can grow on a layer ofgelatin and require the presence of Leukemia Inhibitory Factor (LIF).Human ES cells can grow on a feeder layer of mouse embryonic fibroblasts(MEFs) and may require the presence of basic Fibroblast Growth Factor(bFGF or FGF-2) (See e.g., Chambers I, Colby D, Robertson M, et al.(2003). “Functional expression cloning of Nanog, a pluripotencysustaining factor in embryonic stem cells”. Cell 113 (5): 643-55).

A stem cell, e.g., a human embryonic stem cell, is often defined by thepresence of several transcription factors and cell surface proteins. Forexample, the transcription factors Oct-4, Nanog, and Sox2 form the coreregulatory network that ensures the suppression of genes that lead todifferentiation and the maintenance of pluripotency. The cell surfaceantigens commonly used to identify hES cells are the glycolipids SSEA3and SSEA4 and the keratan sulfate antigens Tra-1-60 and Tra-1-81.

Induced pluripotent stem cells, commonly abbreviated as iPS cells oriPSCs, are a type of pluripotent stem cell artificially derived from anon-pluripotent cell, typically an adult somatic cell, by inducing a“forced” expression of specific genes. Various genes, or a combinationthereof, can be used to induce iPS cells from adult somatic cells. Forexample, Oct-3/4 and certain members of the Sox gene family (Sox1, Sox2,Sox3, and Sox15) can be used to induce iPS cells from adult somaticcells. Additional genes, including certain members of the Klf family(Klf1, Klf2, Klf4, and Klf5), the Myc family (C-myc, L-myc, and N-myc),Nanog, and LIN28, can be used to increase the induction efficiency. Thevarious genes or its encoded proteins can be delivered into the adultsomatic cells by any suitable methods. For example, various genes can bedelivered into the adult somatic cell by a viral transfection system,such as a retroviral system, a lentiviral system, or an adenoviralsystem, or a plasmid without any virus transfection system.Alternatively, proteins encoded by the genes can be delivered into theadult somatic cells directly, e.g., by a repeated treatment of the cellswith certain proteins channeled into the cells via poly-arginineanchors.

The stem cell can be induced to differentiate to form mesoderm by anysuitable treatment or agent. In one example, the stem cell hasdifferentiated to form mesoderm by contacting an undifferentiated stemcell with basic fibroblast growth factor (bFGF), BMP 4 and/or activin A.In another example, the stem cell has differentiated to form mesoderm bycontacting an undifferentiated stem cell with basic fibroblast growthfactor (bFGF), BMP 4 and activin A. The stem cell can be treated withbFGF, BMP 4 and activin A in any suitable order. For example, the stemcell can be differentiated to form mesoderm by contacting anundifferentiated stem cell with basic fibroblast growth factor (bFGF)and BMP 4 before the stem cell is contacted with activin A. In anotherexample, the stem cell can be differentiated to form mesoderm bycontacting an undifferentiated stem cell with wnt-3a (Tran, T. H. et al.Wnt3a-induced mesoderm formation and cardiomyogenesis in human embryonicstem cells. Stem Cells 27, 1869-1878 (2009)), or a small molecule whichacts or functions like wnt-3a, such as Bio or CHIR99021.

Any suitable BMP antagonist can be used in the present methods toenhance cardiac differentiation efficiency of a stem cell. For example,a BMP 4 antagonist can be used. In another example, the BMP antagonistis Noggin. In still another example, the BMP antagonist is Chordin, Tsg,a member of DAN family (Yanagita, M. BMP antagonists: their roles indevelopment and involvement in pathophysiology. Cytokine Growth FactorRev 16, 309-317, (2005)), BMP soluble receptors, such as BMPR1A andBMPR1B, or a small molecule which acts or functions like BMP antagonist,such as Dorsomorphin (Hao, J. et al. Dorsomorphin, a selective smallmolecule inhibitor of BMP signaling, promotes cardiomyogenesis inembryonic stem cells. PLoS One 3, e2904 (2008)).

The present methods can further comprise inhibiting retinoic acidsignaling pathway in the stem cell. The retinoic acid signaling pathwayin the stem cell can be inhibited by any suitable treatment or agent. Inone example, the retinoic acid signaling pathway is inhibited bycontacting the stem cell with a retinoic acid antagonist, a retinoicacid receptor antagonist or a retinoic X receptor antagonist. In anotherexample, the retinoic acid signaling pathway is inhibited by contactingthe stem cell with a pan-retinoic acid receptor antagonist, e.g.,BMS-189453. In still another example, the retinoic acid signalingpathway is inhibited by contacting the stem cell with BMS-453,AGN194310, ANG193109, Ro41-5253, SR11335, 9-cis-retinoic acid, or asmall molecule that inhibits retinoic acid synthesis, such as disulfiramand citral. In yet another example, the retinoic acid signaling pathwayis inhibited by reducing or depleting vitamin A in the culture mediumfor the stem cell.

The present methods can be used to enhance cardiac differentiationefficiency of a stem cell to a suitable degree. In one example, thecardiac differentiation efficiency of the stem cell contacted with theBMP antagonist is at least about 30% higher than the cardiacdifferentiation efficiency of the stem cell not contacted with the BMPantagonist. In specific embodiments, the cardiac differentiationefficiency of the stem cell contacted with the BMP antagonist is atleast about 40%, 50%, 60%, 70%, 80%, 90%, 100%, 2-fold, 3-fold, 4-fold,5-fold, 6-fold, 7-fold, 8-fold, 9-fold, 10-fold, or higher than thecardiac differentiation efficiency of the stem cell not contacted withthe BMP antagonist.

In one specific example, the stem cell is a human embryonic stem cell ora human induced pluripotent stem cell, the BMP antagonist is Noggin andthe cardiac differentiation efficiency of the stem cell contacted withthe BMP antagonist is about 10%, 20%, 30%, 40%, 50%, 60%, 70% or 80%. Inanother specific example, the stem cell is a human embryonic stem cellor a human induced pluripotent stem cell, the BMP antagonist is Nogginand the cardiac differentiation efficiency of the stem cell contactedwith the BMP antagonist is about 60%, 70%, 80%, 90%, 95% or 100%.

The present methods can further comprise contacting the stem cell with awnt inhibitor to differentiate the stem cell into a cardiomyocyte. Anysuitable wnt inhibitor can be used. In one example, the wnt inhibitor isdickkopf homolog 1 (DKK1).

A cardiomyocyte produced by the above methods is also provided.

A composition comprising a stem cell that has differentiated to formmesoderm and treated with an exogenous BMP antagonist is furtherprovided.

C. Methods and Compositions for Promoting Ventricular CardiomyocyteFormation and Ventricular Cardiomyocyte Produced Thereof

In another aspect, the present disclosure provides a method forpromoting ventricular cardiomyocyte formation from a stem cell, whichmethod comprises inhibiting retinoic acid signaling pathway in a stemcell that has differentiated to form mesoderm.

The present methods can be used to promote ventricular cardiomyocyteformation from any suitable stem cell. In one example, the presentmethods can be used to promote ventricular cardiomyocyte formation froma totipotent, pluripotent, multipotent, oligopotent or unipotent stemcell. In another example, the present methods can be used to promoteventricular cardiomyocyte formation from an embryonic stem cell, aninduced pluripotent stem cell, a fetal stem cell or an adult stem cell.In still another example, the present methods can be used to promoteventricular cardiomyocyte formation from a mammalian stem cell such as ahuman stem cell. In yet another example, the present methods can be usedto promote ventricular cardiomyocyte formation from a human embryonicstem cell or a human induced pluripotent stem cell.

The stem cell can be induced to differentiate to form mesoderm by anysuitable treatment or agent. In one example, the stem cell is induced todifferentiate to form mesoderm by contacting an undifferentiated stemcell with basic fibroblast growth factor (bFGF), BMP 4 and/or activin A.In another example, the stem cell is induced to differentiate to formmesoderm by contacting an undifferentiated stem cell with basicfibroblast growth factor (bFGF), BMP 4 and activin A. The stem cell canbe treated with bFGF, BMP 4 and activin A in any suitable order. Forexample, the stem cell can be induced to differentiate to form mesodermby contacting an undifferentiated stem cell with basic fibroblast growthfactor (bFGF) and BMP 4 before the stem cell is contacted with activinA. In another example, the stem cell can be differentiated to formmesoderm by contacting an undifferentiated stem cell with wnt-3a (Tran,T. H. et al. Wnt3a-induced mesoderm formation and cardiomyogenesis inhuman embryonic stem cells. Stem Cells 27, 1869-1878 (2009)), or a smallmolecule which acts or functions like wnt-3a, such as Bio or CHIR99021.

The present methods can further comprise contacting the stem cell with aBMP antagonist to enhance the cardiac differentiation efficiency. Anysuitable BMP antagonist can be used in the present methods. For example,a BMP 4 antagonist can be used. In another example, the BMP antagonistis Noggin. In still another example, the BMP antagonist is Chordin, Tsg,a member of DAN family (Yanagita, M. BMP antagonists: their roles indevelopment and involvement in pathophysiology. Cytokine Growth FactorRev 16, 309-317, (2005)), BMP soluble receptors, such as BMPR1A andBMPR1B, or a small molecule which acts or functions like BMP antagonist,such as Dorsomorphin (Hao, J. et al. Dorsomorphin, a selective smallmolecule inhibitor of BMP signaling, promotes cardiomyogenesis inembryonic stem cells. PLoS One 3, e2904 (2008)).

The retinoic acid signaling pathway in the stem cell can be inhibited byany suitable treatment or agent. In one example, the retinoic acidsignaling pathway is inhibited by contacting the stem cell with aretinoic acid antagonist, a retinoic acid receptor antagonist or aretinoic X receptor antagonist. In another example, the retinoic acidsignaling pathway is inhibited by contacting the stem cell with apan-retinoic acid receptor antagonist, e.g., BMS-189453. In stillanother example, the retinoic acid signaling pathway is inhibited bycontacting the stem cell with BMS-453, AGN194310, ANG193109, Ro41-5253,SR11335, 9-cis-retinoic acid, or a small molecule that inhibits retinoicacid synthesis, such as disulfiram and citral. In yet another example,the retinoic acid signaling pathway is inhibited by reducing ordepleting vitamin A in the culture medium for the stem cell.

In one specific example, the stem cell is a human embryonic stem cell ora human induced pluripotent stem cell, the BMP antagonist is Noggin andthe retinoic acid signaling pathway is inhibited by contacting the stemcell with BMS-189453.

The present methods can further comprise contacting the stem cell with awnt inhibitor to differentiate the stem cell into a ventricularcardiomyocyte. Any suitable wnt inhibitor can be used. In one example,the wnt inhibitor is dickkopf homolog (DKK1).

In one embodiment, the present disclosure provides a method forgenerating a ventricular cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stein cell with an agent, e.g., bFGF and BMP4, to initiate stem cell differentiation; 2) contacting the stem celltreated by the agent, e.g., bFGF and BMP 4, with another agent, e.g.,activin A, to form mesoderm; 3) contacting the stem cell that hasdifferentiated to form mesoderm with a BMP antagonist such as Noggin, toenhance cardiac differentiation efficiency of the stem cell; 4)inhibiting retinoic acid signaling pathway in the stem cell treated byBMP antagonist, e.g., Noggin, to promote ventricular cardiomyocyteformation; and 5) contacting the stem cell treated by BMP antagonist,e.g., Noggin, with a wnt inhibitor such as DKK1 to differentiate thestem cell into a ventricular cardiomyocyte. Ventricular cardiomyocytesproduced by the above method are also provided.

In another embodiment, the present disclosure provides a method forgenerating a ventricular cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with bFGF and BMP 4; 2) contactingthe stem cell treated by bFGF and BMP 4 with activin A; 3) contactingthe stem cell that has been treated by activin A with Noggin; 4)inhibiting retinoic acid signaling pathway in the stem cell treated byNoggin; and 5) contacting the stem cell treated by Noggin with DKK1.Ventricular cardiomyocytes produced by the above method are alsoprovided.

A ventricular cardiomyocyte produced by the above methods is alsoprovided. The ventricular cardiomyocyte can have elevated expressionlevel of a ventricular specific gene, e.g., IRX-4 or MLC-2v, embryonicventricular-like action potentials (AP) and/or Ca²⁺ spark patterntypical of a ventricular cardiomyocyte.

A composition comprising a stem cell that has differentiated to formmesoderm and treated with an exogenous agent that inhibits retinoic acidsignaling pathway in the stem cell is further provided. The exogenousagent can be any suitable agent that inhibits retinoic acid signalingpathway in the stem cell. In one example, the exogenous agent thatinhibits retinoic acid signaling pathway in the stem cell is apan-retinoic acid receptor antagonist, e.g., BMS-189453. In anotherexample, the retinoic acid signaling pathway is inhibited by contactingthe stem cell with BMS-453, AGN194310, ANG193109, Ro41-5253, SR11335,9-cis-retinoic acid, or a small molecule that inhibits retinoic acidsynthesis, such as disulfiram and citral.

D. Methods and Compositions for Promoting Atrial Cardiomyocyte Formationand Atrial Cardiomyocyte Produced Thereof

In still another aspect, the present disclosure provides a method forpromoting atrial cardiomyocyte formation from a stem cell, which methodcomprises stimulating or not inhibiting retinoic acid signaling pathwayin a stem cell that has differentiated to form mesoderm.

The present methods can be used to promote atrial cardiomyocyteformation from any suitable stem cell. In one example, the presentmethods can be used to promote atrial cardiomyocyte formation from atotipotent, pluripotent, multipotent, oligopotent or unipotent stemcell. In another example, the present methods can be used to promoteatrial cardiomyocyte formation from an embryonic stem cell, an inducedpluripotent stem cell, a fetal stem cell or an adult stem cell. In stillanother example, the present methods can be used to promote atrialcardiomyocyte formation from a mammalian stem cell such as a human stemcell. In yet another example, the present methods can be used to promoteatrial cardiomyocyte formation from a human embryonic stem cell or ahuman induced pluripotent stem cell.

The stem cell can be induced to differentiate to form mesoderm by anysuitable treatment or agent. In one example, the stem cell is induced todifferentiate to form mesoderm by contacting an undifferentiated stemcell with basic fibroblast growth factor (bFGF), BMP 4 and/or activin A.In another example, the stem cell is induced to differentiate to formmesoderm by contacting an undifferentiated stem cell with basicfibroblast growth factor (bFGF), BMP 4 and activin A. The stem cell canbe treated with bFGF, BMP 4 and activin A in any suitable order. Forexample, the stem cell is induced to differentiate to form mesoderm bycontacting an undifferentiated stem cell with basic fibroblast growthfactor (bFGF) and BMP 4 before the stem cell is contacted with activinA. In another example, the stem cell can be differentiated to formmesoderm by contacting an undifferentiated stem cell with wnt-3a (Tran,T. H. et al. Wnt3a-induced mesoderm formation and cardiomyogenesis inhuman embryonic stem cells. Stem Cells 27, 1869-1878 (2009)), or a smallmolecule which acts or functions like wnt-3a, such as Bio or CHIR99021.

The present methods can further comprise contacting the stein cell witha BMP antagonist to enhance the cardiac differentiation efficiency. Anysuitable BMP antagonist can be used in the present methods. For example,a BMP 4 antagonist can be used. In another example, the BMP antagonistis Noggin. In still another example, the BMP antagonist is Chordin, Tsg,a member of DAN family (Yanagita, M. BMP antagonists: their roles indevelopment and involvement in pathophysiology. Cytokine Growth FactorRev 16, 309-317, (2005)), BMP soluble receptors, such as BMPR1A andBMPR1B, or a small molecule which acts or functions like BMP antagonist,such as Dorsomorphin (Hao, J. et al. Dorsomorphin, a selective smallmolecule inhibitor of BMP signaling, promotes cardiomyogenesis inembryonic stem cells. PLoS One 3, e2904 (2008)).

The retinoic acid signaling pathway in the stem cell can be stimulatedby any suitable treatment or agent. In one example, the retinoic acidsignaling pathway in the stem cell is stimulated by contacting the stemcell with retinoic acid or vitamin A. In another example, the retinoicacid signaling pathway in the stem cell is stimulated by contacting thestem cell with a retinoic acid receptor agonist, such as LG100268 andLGD1069.

In one specific example, the stem cell is a human embryonic stem cell ora human induced pluripotent stem cell, the BMP antagonist is Noggin andthe retinoic acid signaling pathway is stimulated by contacting the stemcell with retinoic acid or vitamin A.

The present methods can further comprise contacting the stem cell with awnt inhibitor to differentiate the stem cell into an atrialcardiomyocyte. Any suitable wnt inhibitor can be used. In one example,the wnt inhibitor is dickkopf homolog 1 (DKK1).

In one embodiment, the present disclosure provides a method forgenerating an atrial cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with an agent, e.g., bFGF and BMP4, to initiate stem cell differentiation; 2) contacting the stem celltreated by the agent, e.g., bFGF and BMP 4, with another agent, e.g.,activin A, to form mesoderm; 3) contacting the stem cell that hasdifferentiated to form mesoderm with a BMP antagonist such as Noggin, toenhance cardiac differentiation efficiency of the stem cell; 4)stimulating or not inhibiting retinoic acid signaling pathway in thestem cell treated by Noggin to promote atrial cardiomyocyte formation;and 5) contacting the stem cell treated by Noggin with DKK1 todifferentiate the stem cell into an atrial cardiomyocyte. Atrialcardiomyocytes produced by the above method are also provided.

In another embodiment, the present disclosure provides a method forgenerating an atrial cardiomyocyte from a stem cell, which methodcomprises: 1) contacting a stem cell with bFGF and BMP 4; 2) contactingthe stem cell treated by bFGF and BMP 4 with activin A; 3) contactingthe stem cell that has been treated by activin A with Noggin; 4)stimulating or not inhibiting retinoic acid signaling pathway in thestem cell treated by Noggin; and 5) contacting the stem cell treated byNoggin with DKK1. Atrial cardiomyocytes produced by the above method arealso provided.

An atrial cardiomyocyte produced by the above methods is also provided.In one example, an atrial cardiomyocyte can have embryonic atrial-likeaction potentials (AP) and/or Ca²⁺ spark pattern typical of an atrialcardiomyocyte.

A composition comprising a stem cell that has differentiated to formmesoderm and treated with an exogenous agent that stimulates retinoicacid signaling pathway in the stem cell is further provided. Anysuitable exogenous agent can be used to stimulate retinoic acidsignaling pathway in the stem cell. In one example, the exogenous agentthat stimulates retinoic acid signaling pathway in the stem cell isretinoic acid or vitamin A.

E. Pharmaceutical Compositions and Uses of the Cardiomyocytes

The cardiomyocytes can be used for any suitable purposes. In one aspect,the present disclosure provides a pharmaceutical composition fortreating a cardiac injury or disorder, which pharmaceutical compositioncomprises an effective amount of the cardiomyocytes produced by theabove methods, and optionally a pharmaceutically acceptable carrier orexpicient. In one embodiment, the pharmaceutical composition comprises amixture of atrial and ventricular cardiomyocytes. In another embodiment,the pharmaceutical composition comprises at least about 50%, preferably,at least about 60%, 70%, 80%, 90%, 95%, 99%, or 100% atrialcardiomyocytes. In still another embodiment, the pharmaceuticalcomposition comprises at least about 50%, preferably, at least about60%, 70%, 80%, 90%, 95%, 99%, or 100% ventricular cardiomyocytes.

In another aspect, the present disclosure provides a method for treatinga cardiac injury or disorder in a subject, e.g., a human, which methodcomprises administering, to a subject to which such treatment is neededor desirable, an effective amount of the above pharmaceuticalcomposition.

The formulation, dosage and route of administration of thecardiomyocytes, whether predominantly atrial cardiomyocytes,predominantly ventricular cardiomyocytes or a mixture of atrial andventricular cardiomyocytes, preferably in the form of pharmaceuticalcompositions, can be determined according to the methods known in theart (see e.g., Remington: The Science and Practice of Pharmacy, AlfonsoR. Gennaro (Editor) Mack Publishing Company, April 1997; TherapeuticPeptides and Proteins. Formulation, Processing, and Delivery Systems,Banga, 1999; and Pharmaceutical Formulation Development of Peptides andProteins, Hovgaard and Frkjr (Ed.), Taylor & Francis, Inc., 2000;Medical Applications of Liposomes, Lasic and Papahadjopoulos (Ed.),Elsevier Science, 1998; Textbook of Gene Therapy, Jain, Hogrefe & HuberPublishers, 1998; Adenoviruses: Basic Biology to Gene Therapy, Vol. 15,Seth, Landes Bioscience, 1999; Biopharmaceutical Drug Design andDevelopment, Wu-Pong and Rojanasakul (Ed.), Humana Press, 1999;Therapeutic Angiogenesis: From Basic Science to the Clinic, Vol. 28,Dole et al. (Ed.), Springer-Verlag New York, 1999). In specificembodiments, the cardiomyocytes can be combined or formulated withendothelial cells, smooth muscle cells and/or fibroblast cells, andimplanted into a heart. The cell or tissue patch can be transplanted bydirect injection to the infarct area, injection with a catheter orimplanted as a cardio-patch by a surgery. Preferably, the cardiomyocytesare formed from stem cells of the subject that is to be treated. Alsopreferably, the endothelial cells, smooth muscle cells and/or fibroblastcells are also obtained or derived from the subject that is to betreated, e.g., formed from stem cells of the subject that is to betreated.

The cardiomyocytes can be formulated for any suitable route ofadministration. In one example, the cardiomyocytes are administered bysurgery or cell transplantation. The most suitable route in any givencase will depend on the nature and severity of the condition beingtreated and on the nature of the particular cardiomyocytes which arebeing used.

The cardiomyocytes can be administered alone. Alternatively andpreferably, the cardiomyocytes are co-administered with apharmaceutically acceptable carrier or excipient. Any suitablepharmaceutically acceptable carrier or excipient can be used in thepresent method (See e.g., Remington: The Science and Practice ofPharmacy, Alfonso R. Gennaro (Editor) Mack Publishing Company, April1997).

The present method can be used alone. Alternatively, the present methodcan be used in combination with other agent suitable for preventing,treating or delaying a cardiac injury, disease or disorder. Such otheragent can be used before, with or after the administration of thecardiomyocytes. For example, the cardiomyocytes can be co-administeredwith such other agent.

According to the present invention, the cardiomyocytes, alone or incombination with other agents, carriers or excipients, may be formulatedfor any suitable administration route, such as surgery or celltransplantation. The method may employ formulations for administrationin unit dosage form, in ampoules or in multidose containers, with anadded preservative. The formulations may take such forms as suspensions,solutions or emulsions in oily or aqueous vehicles, and may containformulatory agents such as suspending, stabilizing and/or dispersingagents. Alternatively, the active ingredient may be in powder form forconstitution with a suitable vehicle, sterile pyrogen-free water orother solvents, before use.

Pharmaceutically acceptable compositions and methods for theiradministration that may be employed for use in this invention include,but are not limited to those described in U.S. Pat. Nos. 5,736,154;6,197,801 B1; 5,741,511; 5,886,039; 5,941,868; 6,258,374 B1; and5,686,102.

The magnitude of a therapeutic dose in the treatment or prevention willvary with the severity of the condition to be treated and the route ofadministration. The dose, and perhaps dose frequency, will also varyaccording to age, body weight, condition and response of the individualpatient.

It should be noted that the attending physician would know how to andwhen to terminate, interrupt or adjust therapy to lower dosage due totoxicity, or adverse effects. Conversely, the physician would also knowhow to and when to adjust treatment to higher levels if the clinicalresponse is not adequate (precluding toxic side effects).

Any suitable route of administration may be used. Dosage forms includetablets, troches, cachet, dispersions, suspensions, solutions, capsules,patches, and the like. See, Remington's Pharmaceutical Sciences.

In practical use, the cardiomyocytes, alone or in combination with otheragents, may be combined as the active in intimate admixture with apharmaceutical carrier or excipient, such as beta-cyclodextrin and2-hydroxy-propyl-beta-cyclodextrin, according to conventionalpharmaceutical compounding techniques. The carrier may take a wide formof preparation desired for suitable administration. In preparingcompositions for parenteral dosage form, such as intravenous injectionor infusion, similar pharmaceutical media may be employed, water,glycols, oils, buffers, sugar, preservatives, liposomes, and the likeknown to those of skill in the art. Examples of such parenteralcompositions include, but are not limited to dextrose 5% w/v, normalsaline or other solutions. The total dose of the cardiomyocytes, aloneor in combination with other agents to be administered may beadministered in a vial of fluid, ranging from about 1×10³ to 1×10¹⁰cells, e.g., 1×10³, 1×10⁴, 1×10⁵, 1×10⁶, 1×10⁷, 1×10⁸, 1×10⁹, or 1×10¹⁰cells, or any subrange within the range of 1×10³ to 1×10¹⁰ cells.

The invention also provides for kits for carrying out the therapeuticregimens of the invention. Such kits comprise in one or more containerstherapeutically effective amounts of the cardiomyocytes, alone or incombination with other agents, in pharmaceutically acceptable form.Preferred pharmaceutical forms would be in combination with sterilesaline, dextrose solution, or buffered solution, or otherpharmaceutically acceptable sterile fluid. Alternatively, thecomposition may be lyophilized or dessicated; in this instance, the kitoptionally further comprises in a container a pharmaceuticallyacceptable solution, preferably sterile, to reconstitute the complex toform a solution for injection purposes. Exemplary pharmaceuticallyacceptable solutions are saline and dextrose solution.

In another embodiment, a kit of the invention further comprises a needleor syringe, preferably packaged in sterile form, for injecting thecomposition, and/or a packaged alcohol pad. Instructions are optionallyincluded for administration of composition by a physician or by thepatient.

F. Other Uses of the Cardiomyocytes

The cardiomyocytes can be used for any suitable purposes. In oneembodiment, the present disclosure provides a method for identifying amodulator of a cardiomyocyte, which method comprises: 1) contacting acardiomyocyte produced by the above methods with a test substance andmeasuring the effect of the test substance on a property of thecardiomyocyte; 2) measuring the property of the cardiomyocyte notcontacted with the test substance; whereby the property of thecardiomyocyte contacted with the test substance is different from thatof the cardiomyocyte not contacted with the test substance identifiesthe test substance as a modulator, e.g., a stimulator or inhibitor, ofthe property of the cardiomyocyte. In one example, an increase of theproperty of the cardiomyocyte contacted with the test substance relativeto that of the cardiomyocyte not contacted with the test substanceidentifies the test substance as a stimulator of the property of thecardiomyocyte. In another example, a decrease of the property of thecardiomyocyte contacted with the test substance relative to that of thecardiomyocyte not contacted with the test substance identifies the testsubstance as an inhibitor of the property of the cardiomyocyte.

The method can be conducted in any suitable format. Preferably, themethod is conducted in a high-throughput screening (HTS) format.

G. Examples Abstract

Although cell transplantation studies have suggested promisingtherapeutic potentials for myocardial infarction, the incapability toobtain relatively homogeneous ventricular myocytes for transplantationis one major obstacle to the development of clinical therapies formyocardial repair¹. Human embryonic stem cell (hESC) is a promisingsource of cardiomyocytes. Here we report that retinoid signalingregulates the fate specification of atrial versus ventricular myocytesduring cardiac differentiation of hESCs. We found that both Noggin andpan-retinoic acid receptor antagonist BMS-189453 (RAi) significantlyincrease the cardiac differentiation efficiency of hESCs². Investigatingretinoid function by comparing Noggin+RAi-treated cultures withNoggin+RA-treated cultures, our results show that the expression levelof the ventricular-specific gene IRX-4 was radically elevated inNoggin+RAi-treated cultures³, and MLC-2v, another ventricular-specificmarker^(4,5), was expressed in the majority of the cardiomyocytes inNoggin+RAi-treated cultures, but not in those of Noggin+RA-treatedcultures. Flow cytometry analysis and electrophysiological studiesindicated that with 64±0.88% (mean±s.e.m) cardiac differentiationefficiency, 83% of the cardiomyocytes in Noggin+RAi-treated cultures hadembryonic ventricular-like action potentials (AP); while, with 50±1.76%cardiac differentiation efficiency, 94% of those in Noggin+RA-treatedcultures had embryonic atrial-like APs. These results were furtherconfirmed by imaging studies on the patterns and properties of the Ca²⁺sparks of the cardiomyocytes in those two differently treated cultures.These findings demonstrating that retinoid signaling specifies atrialversus ventricular differentiation of hESCs, and relatively homogeneousembryonic atrial and ventricular like myocyte populations can beefficiently derived from hESCs by specifically regulating Noggin andretinoid signals.

Summary

Although cell transplantation studies have suggested promisingtherapeutic potentials for myocardial infarction, the incapability toobtain relatively homogeneous ventricular myocytes for transplantationis one major obstacle to the development of clinical therapies formyocardial repair¹. Human embryonic stem cell (hESC) is a promisingsource of cardiomyocytes. Here we report that retinoid signalingregulates the fate specification of atrial versus ventricular myocytesduring cardiac differentiation of hESCs. We found that both Noggin andpan-retinoic acid receptor antagonist BMS-189453 (RAi) significantlyincrease the cardiac differentiation efficiency of hESCs². Investigatingretinoid function by comparing Noggin+RAi-treated cultures withNoggin+RA-treated cultures, our results show that the expression levelof the ventricular-specific gene IRX-4 was radically elevated inNoggin+RAi-treated cultures³, and MLC-2v, another ventricular-specificmarker^(4,5), was expressed in the majority of the cardiomyocytes inNoggin+RAi-treated cultures, but not in those of Noggin+RA-treatedcultures. Flow cytometry analysis and electrophysiological studiesindicated that with 64±0.88% (mean±s.e.m) cardiac differentiationefficiency, 83% of the cardiomyocytes in Noggin+RAi-treated cultures hadembryonic ventricular-like action potentials (AP); while, with 50±1.76%cardiac differentiation efficiency, 94% of those in Noggin+RA-treatedcultures had embryonic atrial-like APs. These results were furtherconfirmed by imaging studies on the patterns and properties of the Ca²⁺sparks of the cardiomyocytes in those two differently treated cultures.These findings demonstrating that retinoid signaling specifies atrialversus ventricular differentiation of hESCs, and relatively homogeneousembryonic atrial and ventricular like myocyte populations can beefficiently derived from hESCs by specifically influencing BMP andretinoid signaling cascades.

Material and Methods

Maintenance and Differentiation of hESCs.

Undifferentiated hESC line H7 from the WiCell Research Institute wasmaintained on matrigel-coated plates, as previously described³⁷. In thebasic cardiac induction protocol (BP), undifferentiated hESCs wereseeded on gelatin-coated plates at a density of 1-5×10⁵ cells/cm² andcultured with mouse embryonic fibroblast conditioned medium for 3 daysuntil fully confluent. To initiate cell differentiation, the medium waschanged to RPMI1640, supplemented with B27 (Invitrogen). Cells weretreated with 25 ng/ml BMP4 and 6 ng/ml bFGF at day 1, 100 ng/ml activinA at day 2, and 200 ng/ml DKK1 (R&D Systems) from day 6 to day 11. Themedium was changed every 3 days after day 11 (FIG. 1). 250 ng/ml Noggin,1 μM RA (Sigma) or 1 μM RAi were added to the cell culture at the timesspecified in FIGS. 1A-C. Spontaneous beating clusters were typicallyobserved at days 10 to 11. Cardiac differentiation efficiency wasanalyzed on day 14 with CTNT antibody staining and flow cytometry.

Single Cell Preparation of hES-Derived Cardiomyocytes.

Six (60) to 90 day-old differentiated cultures were washed in a low Ca²⁺solution, and then incubated in an enzyme solution for 20 min at 37° C.The dissociation was completed in KB solution by gently shaking for 40min at room temperature. The isolated cells were resuspended in DMEMplus 10% FBS, and transferred on 0.1% gelatin-coated glass coverslips,and then kept in an incubator at 37° C., 5% CO₂. The composition of thelow Ca²⁺ solution was (in mM): 120 NaCl, 5.4 KCl, 5 MgSO₄, 5 Napyruvate, 20 glucose, 20 taurine, 10 HEPES. The pH was adjusted to 7.3with NaOH. KB solution contained (in mM): 85 KCl, 30 K₂HPO₄, 5 MgSO₄, 1EGTA, 2 Na₂ATP, 5 Na pyruvate, 20 glucose, 20 taurine, 5 creatine,adjusted to pH 7.3 with KOH.

Electrophysiological Measurements and Confocal Ca²⁺ Imaging.

Action potentials of cardiomyocytes were recorded in a whole-cell patchclamp configuration, using an Axon 200B amplifier (Axon Instruments) atroom temperature. Data were digitized at 20 kHz and filtered at 2 kHz,and were analyzed by PClamp 9.0. Patch pipettes (2-4MΩ resistance) werefilled with an intracellular solution containing (in mM) 50 KCl, 60K-Aspartate, 1 MgCl₂, 3 Na₂ATP, 10 EGTA, 10 mM HEPES, adjusted to pH 7.3with KOH. Normal Tyrode's solution was used as an extracellular solutionand contained (in mM) 140 NaCl, 5 KCl, 1 CaCl₂, 1 MgCl2, 10 glucose, 10HEPES, adjusted to pH 7.4 with NaOH.

For Ca²⁺ confocal imaging, myocytes were incubated with Fluo-4AM (10μM/L; Molecular Probes) for 10 min at room temperature and then perfusedwith extracellular buffer for about 30 min. Ca²⁺ imaging studies wereperformed on a Leica SP5 confocal microscope equipped with an argonlaser (488 nm) at a magnification of 40× using a 1.25 NA oil immersionobjective. Spontaneous Ca²⁺ sparks and Ca²⁺ transients were recordedusing linescans, obtained at 0.5 ins per line. Images were processed andanalyzed using both MATLAB 7.1 software (MathWorks) and ImageJ(Scioncorp). Detection criteria of 3.8×SD for Ca²⁺ sparks were set, andautomated counting of Ca²⁺ sparks was performed using the Sparkmasterplug-in for ImageJ³⁸.

Flow Cytometry.

Differentiated cell clusters were dissociated into single cells with0.25% trypsin-EDTA, which were then fixed and stained with anti-humanCTNT antibody (R&D Systems) and goat anti-mouse FITC-conjugatedsecondary antibody (Santa Cruz) in PBS plus 0.5% BSA and 0.1% saponin(Sigma) at 4° C. Stained cells were kept in 4% paraformaldehyde forsubsequent quantitative analysis. Data were collected using FACScalibur(Becton Dickinson) and analyzed with FlowJo software (Treestar).

Real-Time RT-PCR.

Total RNA was isolated using Qiagen's RNeasy Plus Mini kit from a singlewell of a 24-well plate of differentiated hES cells. Then 1 μg of totalRNA was reverse transcribed with SuperScript III First-Strand SynthesisSystem (Invitrogen). RT-PCR was carried out using rTaq DNA Polymerase(Takara). Real-time PCR was performed in triplicate using 2× QuantiFastSYBR Green I PCR Master Mix (Qiagen) on a Rotor Gene 6200 Real-Time PCRMachine (Corbett), with an annealing temperature of 60° C. Theexpression of each gene was normalized to GAPDH gene expression. Primersequences are listed in Table 2.

Immunofluorescence.

Sixty (60) day-old differentiated cultures were digested with 0.25%trypsin-EDTA, and the cells were plated on gelatin-coated coverslips for5 days, allowing full attachment to occur. Cells were then fixed in 4%paraformaldehyde, and incubated with primary antibodies of mouseanti-human CTNT (R&D systems), mouse anti-human α-Actinin (Sigma), mouseanti-human β-MHC (ATCC), mouse anti-human MLC-2a (Synaptic Systems), orrabbit anti-human MLC-2v (ProteinTech Group). Goat anti-mouse secondaryantibody conjugated with DyLight 488 (Santa Cruz Biotechnology) and goatanti-rabbit secondary antibody conjugated with Tritc (Santa CruzBiotechnology) were used as needed. After the nuclei were counterstainedwith 4′,6-diamidino-2-phenylindole (DAPI, Sigma), immunofluorescenceimages were visualized and recorded using an Olympus microscope systemX51 or Olympus LSCM FV1000.

Western Blotting.

One well of a 24-well plate of 60 day-old differentiated cells was lysedwith RIPA lysis buffer (Biomiga) for Western blotting. Blots wereincubated with mouse anti-human CTNT, mouse anti-human β-MHC, rabbitanti-human MLC-2v, goat anti-ANF, mouse anti-human MLC-2a, mouseanti-human β-actin, rabbit anti-phospho smad1/5/8 and rabbitanti-samd1/5/8 separately, and then with HRP-conjugated goat anti-mouseor anti-rabbit antibody.

TABLE 1 AP parameters recorded from hESC derived cardiomyoctes. n(cell)Vmax(V/s) APA(mV) APD90(ms) MDP(mV) Nodal-like 6 7.3 ± 4.2 74.5 ± 8.9# 147.6 ± 26.8$ −51.3 ± 8.2† Atrial-like 36 12.8 ± 3.1* 81.6 ± 11.5# 168.8± 26.8$ −55.5 ± 6.5† Ventricular-like 42 11.4 ± 2.8* 86.8 ± 12.4# 285.8± 52.6$ −62.3 ± 8.6† Table 1 AP parameters recorded from hESC-derivedcardiomyoctes. Data are means ± se. n indicates the number of cellstested. Vmax, maximum rate of AP increase; APA. AP amplitude; APD90, APduration measured at 90% repolarization; MDP, maximum diastolicpotential. *P < 0.05 compared with nodal-like; #P < 0.05 compared witheach other; $P < 0.01 compared with each other; and †P < 0.05 comparedwith each other.

TABLE 2 Primer sequences used for qPCR. Gene Forward PrimerReverse Primer Tm NXK2.5 Acctcaacagctccctgactct ataatcgccgccacaaactctcc60° C. CTNT Ttcaccaaagatctgctcctcgct ttattactggtgtggagtgggtgtgg 60° C.IRX4 Ttccgttctgaagcgtggtc tgaagcaggcaattattggtgt 60° C. GAPDHGaaatcccatcaccatcttccagg gagccccagccttctccatg 60° C.Results

Based on the previous studies, we hypothesized that inhibition of BMPpathway after initiation of hESC differentiation and blocking retinoicacid signaling promotes cardiogenesis; Retinoid signaling also regulatesatrial versus ventricular differentiation of hESCs. To test thesehypotheses, we admitted Noggin, RA and its inhibitor RAi to the cardiacdifferentiation cultures at different time intervals, and investigatedtheir effects on cardiogenesis and cardiac subtype specification of hESCderivatives. Our results show that inhibition both BMP and RA signalswith Noggin and RAi significantly promote cardiogenesis, and retinoidsignaling controls the atrial versus ventricular specification ofdifferentiated hESCs. In addition to providing important insights intothe mechanisms that specify cardiac subtypes, our findings alsodemonstrated the direct differentiation of relatively homogeneousembryonic atrial and ventricular-like myocytes from hESCs.

Noggin and RA Antagonist BMS189453 Promote Cardiogenesis ofDifferentiated hESCs.

In order to investigate its functions in cardiac differentiation, Nogginwas systematically added to cardiomyocyte-differentiating hESC culturesgenerated by a protocol developed in our laboratory (see Methods fordetailed description) for different time intervals from days 2 to 5.Results show that cardiac differentiation was slightly repressed whenNoggin was present between days 2 and 3, but significantly promotedbetween days 2.5 and 4.5. Highest cardiac differentiation efficiencieswere achieved between days 4 and 5 (FIG. 1B). Western blot forphosphorylated Smad1,5,8 indicated that Noggin reduced the activities ofBMP signaling (data not shown). Therefore, inhibition of BMP signalingpromotes cardiogenesis in hESCs after the initiation of differentiation.

Previous findings that RA signal restricts embryonic cardiac progenitorraise the possibility that inhibition of RA signaling during cardiacdifferentiation of hESCs could promote cardiogenesis. Vitamin A, thesubstrate for RA synthesis, and RALDH2, the enzyme responsible for RAsynthesis²⁴, were both present in our cultures (data not shown),suggesting the potential of RA signaling activation. We therefore testedthe effects of RA inhibition on promoting hESC cardiac differentiationby adding RAi to our cardiomyocyte-differentiating cultures between days4 and 9 at the time points indicated in FIG. 1C. Flow cytometry showedthat cardiac differentiation was markedly increased when RAi was addedbetween days 6 to 9 (FIG. 1C), demonstrating that inhibition of RAsignaling promotes cardiac differentiation of hESCs.

Next we combined Noggin-treatment on days 4 and 5, and RAi-treatmentfrom days 6 to 8. Flow cytometry of CTNT⁺ cells from day 14 culturesshowed that, with Noggin alone, the differentiation efficiency was50%±3.06% (mean±s.e.m), and that this efficiency increased to 73%±2.08%when cells were treated with both RAi and Noggin (FIG. 1D). This wasconfirmed by results from quantitative RT-PCR analysis of day 14cultures. The expression levels of both CTNT and NKX2.5 weresignificantly higher in Noggin+RAi-treated cultures than in culturestreated with Noggin alone (FIG. 1E). Immunostaining indicated theexpression of typical cardiac markers including CTNT, α-Actinin, MLC-2a,MLC-2v, and p-MHC (FIG. 1F) in cultured cells.

Alternative Retinoid Signals Direct the Differentiation hESCs into TwoDistinct Subtype of Cardiomyocytes.

Because the studies of chicken and mouse embryos indicated that retinoidsignaling regulates the fate specification of in-flow and out-flow tracktissues^(5,20-24), we proposed that the activation or inactivation ofretinoid signaling directs atrial vs. ventricular fate specification ofdifferentiated hESC cardiac progenitors, and that such a mechanism couldbe used to efficiently generate either hESC-derived atrial- orventricular-like myocytes.

To test this hypothesis, either RA or its antagonist, RAi, was added tothe Noggin-treated cultures between days 6 to 8 in parallel experiments(FIG. 1A). After 14 days' differentiation, the percentages of CTNT⁺cells in Noggin+RA and Noggin+RAi-treated cultures was 50.7%±1.76% and64.7%±0.88% respectively (FIG. 2A). Despite there is only about 14%difference in the differentiation efficiencies, the size of beatingcardiomyocytes in the Noggin+RA-treated cultures was smaller than thatof those in the Noggin+RAi-treated cultures (FIG. 2B, D). The beatingrate of cardiomyocytes in the Noggin+RA-treated cultures was also fasterthan that of those in the Noggin+RAi-treated cultures (FIG. 2C and Table1), suggesting that there were two different subtypes of cardiomyocytespresent in these two different cultures. Next, we examined theexpression of ventricular-specific genes, IRX4 and MLC-2v, in the twocultures. Quantitative RT-PCR showed that in Noggin+RAi-treatedcultures, IRX4 expression started to climb on day 8, and by day 14 itwas 10 fold higher than that in the Noggin+RA-treated cultures (FIG.3A). Immunostaining of 60-day-old cultures showed that MLC-2v wasexpressed in the majority of CTNT⁺ cells in Noggin+RAi-treated, but notin Noggin+RA-treated cultures (FIG. 3B), consistent with results fromWestern blotting indicating that although CTNT was expressed at similarlevels in these two cultures, MLC-2v was strongly and only expressed inNoggin+RAi-treated cultures (FIG. 3C). We also compared the expressionof cTNT and MLC-2v in Noggin+RAi-treated, Noggin-treated andNoggin+RA-treated cultures with immunostaining and western blots. Theresults show that Noggin alone treated cultures only about 35% cTNTpositive cells are also MLC-2v positive, and with weak MLC-2v expressiondetected by western blots (FIG. 5). These results indicate that themajority of the cardiomyocytes in Noggin+RAi-treated cultures areembryonic ventricular-like myocytes, whereas the cardiomyocytesdifferentiated in Noggin+RA-treated cultures are either embryonic nodal-or atrial-like myocytes which do not express MLC-2v. We also examinedthe expressions of MLC-2a, and Atrial Nutriation Factor (ANF) in RA andRAi treated cultures with western blots, and results showed that β-MHCis evenly expressed in the two cultures, but MLC-2a and ANF areexpressed with higher level in RA treated cultures than those in RAitreated cultures (FIG. 6).

Electrophysiolgical Characterization Identifies Embryonic Atrial- andVentricular-Like Cardiomyocyte Populations Induced by AlternativeRetinoid Signals.

Due to a lack of endogenous early atrium-specific genetic markers inmammalian systems⁵, we chose to use electrophysiological characters torigorously identify these two cardiac sub-populations. Based on themorphology and classification of AP properties (Table 1)^(6,26) threemajor types of AP (nodal-like, atrial-like, and ventricular-like) wereobserved in our study (FIG. 4A). However, the ratios of the three majortypes of APs were different between Noggin+RA- and Noggin+RAi-treatedcultures; 83% of myocytes (n=23) from the cultures treated withNoggin+RAi possessed ventricular-like APs (FIG. 4A, C), in which theduration of APs could be shortened by application of nifedipine, acalcium channel blocker (FIG. 4B, left), while 94% of myocytes (n=19)from Noggin+RA-treated cultures exhibited an atrial-like AP, and theduration of the AP could not be shortened by nifedipine (FIGS. 4A, Bright and C). These results demonstrate that the majority of thecardiomyocytes in Noggin+RA-treated culture were embryonic atrial-likemyocytes and the majority of those in Noggin+RAi-treated cultures wereembryonic ventricular-like myocytes. Interestingly, in both theNoggin+RA and Noggin+RAi-treated cultures, we did not observe the highpercentages of cardiomyocytes bearing nodal-like APs reported inprevious studies^(6,9).

There are important kinetic differences in Ca²⁺ sparks, the elementaryunit of cardiomyocyte Ca²⁺ signaling, in atrial versus ventricularmyocytes^(27,28). Ca²⁺ sparks are significantly larger and longerlasting in atrial myocytes than those in ventricular myocytes^(28,29).Results of imaging studies indicated that in Noggin+RAi-treated culture87.5% (14/16) of cells tested displayed Ca²⁺ sparks with relatively lowamplitude, fast rise time, short half time decay and small size, typicalCa²⁺ spark properties of ventricular-like myocytes (FIGS. 4D, E). On theother hand, in Noggin+RA-treated cultures, 81.8% (18/22) of cells testeddisplayed Ca²⁺ sparks with higher amplitude, slower rise time, longerhalf time decay and larger size (FIG. 4D, E), suggesting that themyocytes from Noggin+RA-treated cultures were atrial-like myocytes. Thekinetic study of Ca²⁺ release and the ratios of cardiomyocytes bearingthose two patterns in the two differently-treated cultures areconsistent with, and support, previous AP phenotyping-based cardiacsubtype categorization.

Discussion

Our results show that inhibition of BMP signaling after the initiationof cardiac differentiation promotes cardiogenesis of hESCs. This ispartially consistent with the studies on mouse embryonic stem cellsshowing that administrating Noggin before initiation of differentiationpromotes cardiogenesis¹³. Further comparing Noggin treated cells withnon-treated cells revealed that granulocyte colony-stimulating factor(G-CSF) promotes the proliferation of developing cardiomycotes derivedfrom mouse embryonic stem cells³⁰. Noggin sustains undifferentiatedproliferation of hESCs³¹, and BMP4 is required for mouse embryonic stemcell self-renew³². These different self-renewal machineries could be thecause of the differences observed in the cardiac differentiation studiesof human and mouse embryonic stem cells.

Even though western blots revealed the differential expressions of ANFand MLC-2a, both genes are expressed in 60 day old Noggin+RA- andNoggin+RAi-treated cultures (FIG. 6). This is consistent with Dr.Rosenthal's statement, which indicated that there is no early atrialspecific marker in mouse system. Instead, they used a proximal 840 bpquail SMyHC3 promoter to label the sino-atrial tissue from the earlieststage of heart developing⁵.

Previous studies of chicken and mouse embryos have proposed that RAsignaling determines sinoatrial cell fate, whereas ventricular fate isspecified in the absence of RA²⁰. Our study shows that blocking RAsignaling induces the expression of the ventricle-specific marker MLC-2vin major hESC-derived cardiomyocytes, and that these cells possess APsand Ca²⁺ sparks typical of ventricular myocytes. Exogenous RA treatmentdirects the differentiation of hESCs into myocytes which retaincharacteristic atrial-like APs and large Ca²⁺ sparks or Ca²⁺ transients.Our results demonstrate that the activation or inhibition of retinoidsignals instructs atrial versus ventricular specification ofdifferentiating hESCs. Different from our results, previous study showsthat RA enhances development of ventricular cardiomyocytes derived frommouse embryonic stem cells³³. This could represent the differences onthe differentiation culture systems used in the two studies, embryoidbody procedure versus flat culture system, and the timing when RA isadmitted.

Potential risk of ventricular arrhythmias caused by the heterogeneity ofhESC-derived cardiomyocytes is one of the major hurdles for applicationof hESCs to cardiac repair^(1,6,10). Application of relativelyhomogeneous ventricular myocytes derived from hESCs in myocardial repairhas great potential to reduce this risk, removing one of the majorbarriers for developing hESC based myocardial repair strategy. Recentadvance in tissue engineering, which shows that mouse ventricularprogenitor cells can be isolated from mouse embryonic stem cellderivatives using a genetic labeling approach, and are used to generatefunctional ventricular muscles³⁴, suggests that human functionalventricular heart muscles can be generated with embryonicventricular-like myocytes directly differentiated from hESCs. With achemical defined culture system and no genetic manipulation, the directdifferentiation procedures we developed would be easy to employed inclinical studies of myocardial repair. Another challenge for developinghESC based myocardial repair is to develop the biotechnology for rapidlygenerating the large amounts of ventricular myocytes needed fortransplantation. Our study demonstrated the efficient differentiation ofembryonic atrial- and ventricular-like myocytes from hESCs byeliminating embryoid body procedure, a time consuming step which iscommonly used in cardiac differentiation of embryonic stem cells. Ifcombined together with induced pluripotent stem (iPS) celltechnology^(35,36), programmed differentiation of atrial- andventricular-like myocytes could be used to develop not only safe cellsources for personalized cardiac repair, but could also provide cellularmodels for the study of genetic atrial or ventricular diseases.

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Citation of the above publications or documents is not intended as anadmission that any of the foregoing is pertinent prior art, nor does itconstitute any admission as to the contents or date of thesepublications or documents.

What is claimed is:
 1. An in vitro method for enhancing cardiacdifferentiation efficiency of a mammalian mesodermal cell, the methodcomprising: contacting a mammalian mesodermal cell with a bonemorphogenetic protein (BMP) antagonist and inhibiting retinoic acidsignaling pathway in the mammalian mesodermal cell contacted with theBMP antagonist, thereby enhancing cardiac differentiation efficiency ofthe mammalian mesodermal cell.
 2. The method of claim 1, wherein themammalian mesodermal cell is differentiated from a human embryonic stemcell or a human induced pluripotent stem cell.
 3. The method of claim 1,wherein the mammalian mesodermal cell is generated by contacting anundifferentiated mammalian stem cell with basic fibroblast growth factor(bFGF), BMP 4 and/or activin A.
 4. The method of claim 1, wherein theBMP antagonist is a BMP4 antagonist, Noggin, Chordin, Tsg, a BMP solublereceptor, BMPRIA, BMPRIB or Dorsomorphin.
 5. An in vitro method forpromoting ventricular cardiomyocyte formation from a mammalianmesodermal cell, the method comprising inhibiting retinoic acidsignaling pathway in a mammalian mesodermal cell, thereby promotingventricular cardiomyocyte formation from the mammalian mesodermal cell.6. The method of claim 5, wherein the mammalian mesodermal cell isdifferentiated from a human embryonic stem cell or a human inducedpluripotent stem cell.
 7. The method of claim 5, wherein the mammalianmesodermal cell is generated by contacting an undifferentiated mammalianstem cell with basic fibroblast growth factor (bFGF), BMP 4 and/oractivin A.
 8. The method of claim 5, further comprising contacting themammalian mesodermal cell with a BMP antagonist, wherein the BMPantagonist is a BMP4 antagonist, Noggin, Chordin, Tsg, a BMP solublereceptor, BMPRIA, BMPRIB or Dorsomorphin.
 9. The method of claim 5,wherein the retinoic acid signaling pathway is inhibited by contactingthe mammalian mesodermal cell with a pan-retinoic acid receptorantagonist, a retinoic acid antagonist, a retinoic acid receptorantagonist, or a retinoic X receptor antagonist.
 10. An in vitro methodfor promoting atrial cardiomyocyte formation from a mammalian mesodermalcell, the method comprising stimulating retinoic acid signaling pathwayin a mammalian mesodermal cell, thereby promoting atrial cardiomyocyteformation from the mammalian mesodermal cell.
 11. The method of claim10, wherein the mammalian mesodermal cell is differentiated from a humanembryonic stem cell or a human induced pluripotent stem cell.
 12. Themethod of claim 10, wherein the mammalian mesodermal cell is generatedby contacting an undifferentiated mammalian stem cell with basicfibroblast growth factor (bFGF), BMP 4 and/or activin A.
 13. The methodof claim 10, further comprising contacting the mammalian mesodermal cellwith a BMP antagonist, wherein the BMP antagonist is a BMP4 antagonist,Noggin, Chordin, Tsg, a BMP soluble receptor, BMPRIA, BMPRIB orDorsomorphin.
 14. The method of claim 10, wherein the retinoic acidsignaling pathway in the mammalian mesodermal cell is stimulated bycontacting the cell with retinoic acid.
 15. An in vitro method forgenerating a ventricular cardiomyocyte from a stem cell, the methodcomprising: 1) contacting a mammalian stem cell with bFGF and BMP4 toinitiate stem cell differentiation; 2) contacting the mammalian stemcell treated with bFGF and BMP 4 with activin A to form a mesodermalcell; 3) contacting the mammalian mesodermal cell with Noggin for atleast two days to enhance cardiac differentiation efficiency of themammalian mesodermal cell; 4) inhibiting retinoic acid signaling pathwayin the mammalian mesodermal cell of step 3) for at least three days topromote ventricular cardiomyocyte formation and concurrently contactingthe mammalian mesodermal cell of step 3) with DKK1 for at least six daysto differentiate the mammalian mesodermal cell into a ventricularcardiomyocyte.
 16. An in vitro method for generating an atrialcardiomyocyte from a stem cell, the method comprising: 1) contacting amammalian stem cell with bFGF and BMP4 to initiate stem celldifferentiation; 2) contacting the mammalian stem cell treated with bFGFand BMP 4 with activin A to form a mesodermal cell; 3) contacting themammalian mesodermal cell with Noggin for at least two days to enhancecardiac differentiation efficiency of the mammalian mesodermal cell; 4)stimulating retinoic acid signaling pathway in the mammalian mesodermalcell of step 3) for at least three days to promote atrial cardiomyocyteformation and concurrently contacting the mammalian mesodermal cell ofstep 3) with DKK1 for at least six days to differentiate the mammalianmesodermal cell into an atrial cardiomyocyte.
 17. The method of claim 1,wherein the mammalian mesodermal cell is generated by contacting anundifferentiated mammalian stem cell with Wnt-3a, Bio, or CHIR99021. 18.The method of claim 5, wherein the mammalian mesodermal cell isgenerated by contacting an undifferentiated mammalian stem cell withWnt-3a, Bio, or CHIR99021.
 19. The method of claim 10, wherein themammalian mesodermal cell is generated by contacting an undifferentiatedmammalian stem cell with Wnt-3a, Bio, or CHIR99021.
 20. The method ofclaim 5, wherein the retinoic acid signaling pathway is inhibited byreducing vitamin A in a culture medium comprising the mammalianmesodermal cell.
 21. The method of claim 5, wherein the retinoic acidsignaling pathway is inhibited by depleting vitamin A in a culturemedium comprising the mammalian mesodermal cell.
 22. The method of claim10, wherein the retinoic acid signaling pathway in the mammalianmesodermal cell is stimulated by contacting the cell with vitamin A. 23.An in vitro method promoting atrial cardiomyocyte formation from amammalian mesodermal cell, the method comprising not inhibiting retinoicacid signaling pathway in a mammalian mesodermal cell, thereby promotingatrial cardiomyocyte formation from the mammalian mesodermal cell. 24.An in vitro method for generating an atrial cardiomyocyte from a stemcell, the method comprising: 1) contacting a mammalian stem cell withbFGF and BMP4 to initiate stem cell differentiation; 2) contacting themammalian stem cell treated with bFGF and BMP 4 with activin A to form amesodermal cell; 3) contacting the mammalian mesodermal cell with Nogginfor at least two days to enhance cardiac differentiation efficiency ofthe mammalian mesodermal cell; 4) not inhibiting retinoic acid signalingpathway in the mammalian mesodermal cell of step 3) for at least threedays to promote atrial cardiomyocyte formation and concurrentlycontacting the mammalian mesodermal cell of step 3) with DKK1 for atleast six days to differentiate the mammalian mesodermal cell into anatrial cardiomyocyte.